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Full text of "Handbook Of Chemical Engineering - I"

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350                              CHEMICAL ENGINEERING
Sluicing is the cheapest where water and ample disposal room are available while shoveling or mechanical excavators are used where they are not. Percolation leaching is done on ores crushed as fine as 100 mesh and as coarse as 1 inch. Diffusion Process in Beet-sugar Manufacture.—In leaching beet sugar the percolation method is used universally. It would be very difficult to handle this material in any other way and the development of labor-saving machinery has reduced operating costs tremendously. A battery is made up of 12 diffusors or cells. A diffusor has a capacity of about 42 to 86 bu. and its height is about twice its diameter. A grating made up of a perforated sheet-iron plate holds the sliced beets in the diffusor and when the dissolution is complete they are discharged through a discharge gate in the side near the bottom of a diffusor, being charged into the cell through a cover over the top of the diffusor.
The dissolution is effected by continuous counter-current leaching, the liquors being fed in at the bottom of the diffusors passing through the beets to the next, etc. Before placing a diffusor in the system therefore it is first necessary to drive out all the air. This is done by opening an air cock in the top of the tank and feeding the juice from the preceding diffusor at the bottom. When the juice reaches the level of the top of the diffusor the air cock is closed and the direction of the flow reversed. When the system has become balanced a calculated amount of the concentrated juice will be withdrawn from, the system before the juice is run through the fresh charge.
The solution from each diffusor passes to heaters operated so that the temperature of the liquor rises from 35°C. for the juice leaving the sliced beets getting the fresh water to 70 to 75° for the strong liquors leaving the last diffusor containing the fresh beet cossettes. The amount of water required is 1.2 to 1.5 the weight of the beets producing 48 to 55 liters of juice per hectoliter of beets. The exhausted beets contain 0.5 per cent of sugar and about 95 per cent water, the original content having been about 15 to 18 per cent sugar and 75 to 86 per cent water or 94 per cent extraction.
The mechanism of diffusion of the sugar through the membranes surrounding the cells which contain the sugar depends upon the action of osmosis. There is a tendency for the sugar particles to pass through the membrane and into the surrounding solution (exosmosis) but there is also a tendency for water to pass from the solution through the membrane to the cell (endosmosis). These two counter actions will reach a balance when the strength of the solutions within and outside of the cells are the same. Therefore to obtain a complete extraction the solution outside the cells must be made progressively less concentrated. (The sugar in the beet being in solution from the start, the problem is not one of dissolution but of washing the solution from the solids and agitation is therefore not necessary.) The membrane around the cells of the beets as added to the diffusor, however, is covered with a film of protoplasm through which the diffusion is very slow and the solution is heated in order to coagulate this and thereby render it permeable.
Preparation of Organic Extracts.—Tanning extracts, etc. are usually prepared by leaching tan bark, etc. by percolation. Here as in the case of the diffusion process in beet-sugar manufacture there is no dissolution but from the start the process is a process of washing only. This offers no material difference in the procedure but limits the strength of the strong liquors to the strength of the original solution of this material in the plant cells. If the material were present in a crystalline form it would be possible to obtain saturated solutions from the percolation vats. This is far from being the case, however, in both the beet sugar and tanning processes,